<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- 打印出1000-2000年中所有的闰年，并以每行四个数的形式输出 -->
    <script>
        var a =1000;
var b = 0;
while(a<=2000)
{
// 闰年判断条件
if((a%400==0)||(a%4==0)&&(a%100!==0))
{
b++;
document.write(a+`&ensp;`);
if(b%4==0)
{
document.write(`<br>`);
}
}
a++;
}
    </script>
</body>
</html>